a) Field of the Invention
The present invention relates to high frequency semiconductor devices, and more particularly to high frequency semiconductor devices with a high frequency semiconductor IC chip which is face-down-bonded to a dielectric circuit board.
b) Description of the Related Art
Monolithic microwave integrated circuits (MMIC) are known as semiconductor IC devices used in a high frequency band such as microwaves and millimeter waves. Although a high frequency band is preferentially used for signal transmission and reception, some signal processing and other operations may be performed in another frequency band not so high in many cases. In such a case, each circuit unit operating at a high frequency band has been formed in an IC chip and bonded to a circuit board. Most of MMIC's are made of such IC chips.
The mobility of electric charge carried in a semiconductor is limited by the material of semiconductor. The carrier mobility in Si which is used most often as the material of semiconductor devices, is generally lower than that in compound semiconductor such as GaAs and InP. Compound semiconductor is therefore more suitable for the material of high frequency IC's than Si. Compound semiconductor is also suitable for high frequency circuits because it can form semuinsulating regions therein. A compound semiconductor substrate can hence form semiconductor devices having very low junction capacitances. Most MMIC's are therefore made of compound semiconductor chips although these chips are very expensive.
Millimeter band circuits are used, for example, in radars operating at 77 GHz. If all circuits of a radar are formed on a compound semiconductor substrate, the radar becomes very expensive because the area occupied by a transmitter antenna becomes large. In order to realize a radar of low cost and high performance, it is desired that an antenna is formed on a low cost substrate to which an MMIC made of compound semiconductor is electrically connected.
Electrical connection of an MMIC to a dielectric substrate formed with transmission lines, however, poses some problems. The simplest way to the electrical connection is to bond the bottom surface of an MMIC to a dielectric substrate and connect terminals on the top surface of the MMIC to the transmission lines formed on the dielectric substrate by some interconnection means such as wires and tapes. However, if this interconnection means has inductance, a flow of high frequency current is hindered. It is therefore undesirable to use a narrow and long interconnection means.
A more preferred way is to face-down-bond an MMIC, with its circuit surface turned upside down, to a dielectric substrate formed with transmission lines. With this method, interconnection means can be shortened and inductance thereof can be reduced. Electrical connection to a circuit can be realized by forming bumps, pillars, metal plates, or the like, between MMIC and lines on the dielectric substrate.
Stray or parasitic capacitance formed on a circuit operating at a high frequency poses significant issues. Capacitance formed between a line and a nearby conductor allows a high frequency leak current to flow therebetween. It is therefore desired that a separation area is made of material having as small a dielectric constant as possible. Media having a lowest dielectric constant are gas, such as air, and vacuum. In this connection, face-down-bonding of MMIC provides an ample possibility of achieving high performance.
A sufficiently strong support force is, however, difficult to obtain if an MMIC is supported only by bumps, pillars or metal plates. Vibrations or even light impacts may dismount the MMIC from a dielectric substrate.
In order to solve the above problems, a method has been proposed in which reinforcing resin is filled in between an MMIC and a dielectric substrate. For example, light setting type resin is filled in and cured with light transmitted through a dielectric substrate. With this structure, an MMIC can be held with a sufficient support force.
Although the support force is ensured by filling in reinforcing resin between a face-down-bonded MMIC and an MMIC mounting dielectric substrate, the electrical characteristics of the MMIC are degraded.